Gas sensor arrangements for detecting a wide variety of analytes, for example methane or carbon dioxide, are well known. Examples of such gas sensors are disclosed in EP 0 616 207 A2, WO 00/55603 A1, and DE 199 251 96 C2. The gas sensors comprise a measuring radiation source, a gas measuring chamber, and a radiation detector. The gas sensors are based on the principle that a large number of polyatomic gases absorb radiation, particularly in an infrared wavelength range. The absorption occurs at a wavelength characteristic for the gas, for example at 4.24 μm for carbon dioxide. Using gas sensors, it is therefore possible to detect the existence of a gas component and/or the concentration of the gas component in a measuring gas. The intensity of the radiation measured by the radiation detector is, in accordance with the known Beer-Lambert law, a measure of the concentration of the measuring gas. In a non-dispersive infrared (NDIR) sensor, for example, a broadband radiation source may be used and the wavelength of interest adjusted via an interference filter or grid. Alternatively, a selective radiation source may be used, for example a light-emitting diode or a laser, in combination with non-wavelength-selective radiation receivers.
The use of gas sensor arrangements to detect, for example, carbon dioxide is important in a large number of applications. For example, gas sensor arrangements can be used to monitor and regulate the quality of interior air, the cleaning cycle of self-cleaning ovens, the supply of carbon dioxide to plants in greenhouses, and the breathing air of a patient. Additionally, gas sensor arrangements can be used in warning systems to detect, for example, leaking carbon dioxide, such as in air conditioning systems.
In the automotive field, gas sensor arrangements can be used, for example, to monitor the carbon dioxide content of the interior air to increase the energy efficiency of the heating and air conditioning systems. For example, to increase energy efficiency during heating and air conditioning, the carbon dioxide content of the air in the interior of the vehicle is monitored. In the event that an increase in carbon dioxide concentration occurs, a supply of fresh air is introduced via a fan flap. Additionally, modern air conditioning systems are based on carbon dioxide coolants. The gas sensors can therefore fulfil a monitoring function in conjunction with issuing carbon dioxide in the event of potential defects. Gas sensors of this type, however, must meet stringent requirements with respect to ruggedness, reliability, and miniaturization.
One example of a gas sensor arrangement is shown in EP 0 616 207 A2. In this gas sensor arrangement, the measuring radiation source is not operated uniformly but pulsed with a specific frequency. A constant frequency and a specific pulse duty factor are normally selected. The pulse duty factor identifies the ratio of operating time (pulse width) to periodic time. A narrow band filter in the radiation detector region can reduce interference during signal processing. The frequency of the filter corresponds to the pulse frequency with which the measuring radiation source is pulsed.
These types of gas sensor arrangements have a crucial disadvantage in that when the measuring radiation source is switched-on, the settling time or period until usable measurement results are available is relatively long, because only a relatively small amount of energy is radiated per pulse. This is also the case in gas sensor arrangements having operating modes, where the measuring radiation source emits no radiation for a relatively long period.
For example, FIG. 2 shows a conventional uniform pulse sequence for a conventional gas sensor arrangement with a pulse duty factor t1/T of 0.16, a periodic time of T=2.5 seconds, and a pulse width of 0.4 seconds. At time t=0, the conventional gas sensor arrangement is switched-on and the measuring radiation source begins to emit pulses according to curve 201 (a bottom level of the curve 201 indicates the switched-off state and a top level of the curve 201 indicates a switched-on state). After being switched-on, however, the conventional gas sensor arrangement has to settle thermally, which takes about 10–15 measurement values. Actual measurements, therefore, can not begin until the settled state is reached at about time t=tm. Thus, the first 10–15 measurement values are not usable. This is particularly problematic in safety applications, where the gas sensor arrangement has to be frequently switched-on and off.
Another example of a gas sensor arrangement is shown in DE 199 25 196 C2. In this gas sensor arrangement, a reference radiation source is provided in addition to a measuring radiation source. The reference radiation source is switched-on periodically to monitor the aging of the measuring radiation source. The reference radiation source is not used for normal measurement, but is operated at large monitoring intervals for a short duration to detect the aging of the measuring radiation source. The operation of the reference radiation source at large monitoring intervals for only a short duration is necessary so that the aging of the reference radiation source is disregarded.
In these types of gas sensor arrangements, the settling time or period until usable measurement results are available is also relatively long after having been switched-off. For example, FIG. 7 shows a conventional pulse sequence for a reference radiation source and a measuring radiation source. Curve 701 denotes the pulse sequence for the measuring radiation source and curve 702 denotes the pulse sequence for the reference radiation source. In principle, the reference radiation source is only switched-on for referencing. In FIG. 7, the reference radiation source is switched-on at time t=tr. When the reference radiation source is first switched-on, it requires a certain amount of time until thermal equilibrium is established and reliable measurement values can be delivered.
At time t=tm, the measuring radiation source is again switched-on to continue measurement and at the same time to detect comparative values for correction. Because the measuring radiation source was switched-off during the time in which the reference radiation source was pulsed, the settled state for the measuring radiation source must also now be re-established.
It is commonly assumed that approximately four measurement values are required to reach the settled state and approximately four measurement values are required for referencing for a total of eight pulses. The referencing therefore lasts for 16 measuring cycles with unaltered pulse sequences being emitted. If approximately three seconds are calculated, for example, per measuring cycle, the entire referencing process lasts at least 48 seconds. There is therefore a total of 48 seconds during which no measurement data which can be utilized in a warning system. As a result, a considerable amount of gas could escape unnoticed through a gas leak during the referencing phase.